THE GROWTH FORM OF POPULATIONS 



317 



librium is maintained that displays a 

 minimum of variability. This is a good ex- 

 ample. The points cluster near the mean 

 line; the line is practically horizontal; none 

 of the four populations deviate substantially 

 from the curve; the observations cover a 



gests that this drop was associated with 

 four factors: "(a) the prevalence of sheep 

 scab, which necessitated the passing of the 

 'Scab Act of 1870;' (b) the prevalence of 

 fluke in certain pasture areas; (c) the de- 

 velopment of the rabbit pest, which neces- 



Fig. 



1000 



HOURS 

 109. The equilibrium maintained by a laboratory population of yeast for approximately 

 1000 hours. (From Richards.) 



long enough period of time to have validity 

 as a trend. 



Populations of Domesticated Animals 



Davidson (1938a) reports a long period 

 of equilibrium for the sheep population of 

 Tasmania, extending from 1859 to 1924. 

 This is shown in Figure 104, earlier dis- 

 cussed in connection with logistic growth, 

 in which an equilibrium Hue is fitted to the 

 observed points. There is some obvious 



sitated the passing in 1871 of 'An Act to 

 Provide for Destruction of Rabbits in Tas- 

 mania;' (d) the persistent fall in the price 

 of wool which dropped from 22d. a pound 

 to 15d. a pound between 1862 and 1870." 

 By 1874 the population had reconstituted 

 itself. 



Natural Populations 



A semiequilibrium position in which the 

 population maintains a low level with min- 



SEPT- 

 DEC 



JANr 



FEB. 



MARCH-' SEPT- 

 AUG. DEC. 



JAN.- MARCH-' SEPT- 

 FEB. AUG. DEC 



Fig. 110. Fluctuations in numbers of the diatom, Rhizosolenia styliformis. The ordinate scale 

 between zero and 100 is exaggerated. ( From Lucas. ) 



variation between points and curve, and 

 this case also would be suitable as an illus- 

 tration of moderate fluctuations. However, 

 the equilibrium hne actually remains essen- 

 tially horizontal for the sixty-five year 

 period, attesting to considerable stability by 

 the population. The curve shows that in 

 1870 there was a considerable drop in 

 population size; this is the greatest depar- 

 ture from the equilibrium. Davidson sug- 



imum variabihty for a considerable period 

 is described by Lucas (1940) for the dia- 

 tom Rhizosolenia stijliformis. This is a small 

 fragment of a comprehensive study of 

 oceanic plankton populations, the objec- 

 tives and general plan of which have been 

 set forth by Hardy (1939). This group of 

 English investigators used an ingenious de- 

 vice called "the continuous plankton re- 

 corder," which is drawn by ships as they 



